do you have d2x cIOS installed? (I guess you do, because USBLoader is doing a check before launching a game). which version ?
You probably followed a bad hacking guide, which told you to install d2x r53 because "LAST !! should be better !"
But, no, it's wrong. r53 doesn't work with EmuNAND. r53 is a beta version, used only for a specific purpose (USB port1 support, which you don't need) but break other features.
Reinstall d2x with r52 instead !
Did you try different games? Maybe the one you try to launch is bad/corrupted/or doesn't work ?
verify your SD cad is FAT32 and MBR.
there's no check in the loader as it expect users to have these settings.